The invention relates to a polymeric membrane fuel cell stack. Fuel cells are electrochemical generators of direct electric energy; in other words, they convert the free energy of reaction of a fuel (for instance a gaseous mixture containing hydrogen, or light alcohols such as methanol or ethanol) with an oxidant (for instance air or oxygen) without degrading it completely to thermal energy, and therefore without suffering the limitations of Carnot's cycle. Such conversion is achieved through the electrochemical oxidation of the fuel at the negative pole of the cell, with the consequent release of H+ ions, while the oxidant is reduced at the positive pole, wherein such H+ ions are consumed; the migration of said ions from the negative to the positive compartment of the cell takes place through a suitable electrolyte. In the case of polymeric membrane cells, such an electrolyte is constituted of a cation exchange membrane, acting at the same time as a physical separator for the gaseous reactants and as a positive ion conductor, meanwhile presenting a substantially nil electronic conduction, so as to maximise the electromotive force of the generator. The aforementioned half-reactions of fuel oxidation and of oxidant reduction are normally carried out with the help of a catalyst, in intimate contact with the respective sides of the polymeric membrane.
Fuel cells are considered to be an excellent alternative to the traditional systems of electric generation, mostly due to the extremely favourable environmental impact (absence of noxious emissions and noise, release of water as by-product); they are employed both in the field of stationary power generation of various size (electric plants, continuity or emergency power generators, etc.) as well as in the field of mobile applications (electric automotive applications, generation of automotive energy or auxiliary energy for space, submarine and naval applications).
The polymeric membrane fuel cells offer, compared with other fuel cells, further advantages, due to the quick start-up and achievement of the optimum operation conditions, the high power density, the intrinsic reliability connected both to the absence of moving parts and to the absence of corrosion phenomena and heavy thermal cycles from the point of view of materials; in fact among all the fuel cells of the prior art, the polymer electrolyte fuel cells exhibit in absolute the lowest operating temperature (usually, 70-100° C.).
The advantages deriving from the possibility of operating in such conditions go nevertheless together with some limitations, such as the lower activity of the catalysts at low temperature, the extreme vulnerability of the same to carbon monoxide, inevitably present in the hydrogen based fuel mixtures coming from the usual industrial processes, the impossibility of exploiting the generated heat by co-generation systems.
For this reason, one of the most recent trends in this technical field lays in the development of polymeric membranes able to operate at slightly higher temperature (100-150° C.), so as to remarkably reduce the aforementioned drawbacks, without incurring the far heavier ones, in terms of thermal cycles, corrosion phenomena, physical properties of the construction materials, which hinder the use of the high temperature fuel cells (200-800° C.) for low and medium size applications.
Another current trend is the development of membranes capable of working at higher current density at the same cell voltage, i.e. at the same level of electric efficiency of the system. The increase in current density allows in fact to raise the power and energy density of the fuel cell stacks, thereby decreasing the material cost per unity of installed power. This is a very important point for the industrial success of this application, still affected by costs which are not always competitive with those of rival technologies.
A last trend to be taken into consideration concerns the development of fuel cell stacks with gaseous reactants at near-ambient pressure, to decrease the operating costs by using ambient or slightly compressed air. All of the above listed factors lead to increasingly demanding system requirements as far as fuel cell water and thermal management is concerned, those already being two of the most delicate and important aspects in the design of such devices.
It is well known that the former aspect, namely the fuel cell water management, is complicated by two contrasting requirements associated with two vital elements of the system: the ion-exchange membrane, in fact, must be kept in a sufficiently hydrated state so as to develop an adequate ionic conductivity, thereby preventing the onset of resistive penalties that would severely affect the electric efficiency, until completely hindering, in the worst of the cases, the operation of the generator. On the other hand, the catalytic sites on which the two half-reactions take place must be continuously supplied with gaseous reactants for the whole process to be sustained. Such gaseous supply to the catalytic sites can be correctly achieved only if the latter are not covered with a layer of excess liquid water capable of isolating the same, because the diffusion of the gas through such a layer would be so slow that diffusive penalties would arise, again severely affecting the electric efficiency of the whole system.
The water management must take into account the water produced at the positive pole by the cathodic half-reaction, the fraction of the same which migrates by diffusion from the positive to the negative compartment, the water transported by the H+ ions in the hydration shell, and the water withdrawn from the system through the gaseous exhaust outlet. An adequate amount of water must be consequently supplied to each compartment of the cells together with the reactants, in order to compensate for the difference between the overall amount withdrawn and the amount produced. The adoption of more severe process conditions, in terms of operating temperature and of generated current density (the latter being a further source of local heating and increased water evaporation from the ion-exchange membrane) requires, in particular, an increment in the quantity of water to be supplied to the system, which is not easily achieved especially at low pressure, wherein the increased expansion of the gasses increases their volumetric flow at a given molar flow, thereby rising also the capability of the gas flow of withdrawing water from the cells.
The overall thermal balance of a fuel cell is less complex, nevertheless it is associated with technological issues of no lesser extent. The amount of heat produced in a fuel cell operating at high current density is remarkable, and it must be effectively withdrawn to achieve a stable operating temperature, otherwise it would also affect the water management of the system, with a synergistic effect that is not easily reversed after its onset. Moreover, when high temperatures are reached even locally and for a limited time, a structural failing of the membranes, which are among the most expensive components of the whole apparatus, and which are quite difficult to replace without damaging other components of the stacks, can easily occur. It is therefore mandatory to provide an effective system for the heat removal, and the latter, in its turn, should not impart high mass and volume constraints, which would hamper the competitiveness of the system especially for certain kinds of applications (such as in the electric automotive field).
A useful means to temper the extent of problems in heat withdrawal is the selection of materials with a sufficiently high thermal exchange coefficient for fuel cell manufacturing: U.S. Patent 5,482,792,for instance, discloses fuel cell stacks provided with bipolar plates, end-plates and current collectors, whose surfaces are utilized to sustain the thermal exchange, made of metallic materials of various kinds (aluminum and nickel alloys, steels,and so on). The use of high surface reticulated materials as current collectors (metallic foams, sintered materials, overlapping of meshes or expanded sheets) also allows the coupling of the two functions of humidifying the gaseous reactants and withdrawing the generated heat, as disclosed in U.S. patent application Ser. No.: 09/937,973 filed Oct. 1, 2001 now U.S. Pat. No. 6,835,477 wherein fuel cell stacks provided with a dedicated circuit for the injection of water inside single cells equipped with current collectors of this kind are described. The heat is thus withdrawn through the partial evaporation taking place on the material with high specific surface, utilizing the residual fraction of water which is not evaporated, for humidifying the membranes. Such device permits the operation of fuel cell stacks at a higher current density with respect to a conventional system, which may for instance consist in the pre-humidification of the reactants through bubbling the same in appropriate pressurized tanks, while supplying a separate water flow in an independent circuit through suitable grooves provided in the linear part of each bipolar plate.
Such water flows heat withdrawal through the surfaces of such bipolar plates. Not withstanding the manufacturing simplification introduced by the design disclosed in U.S. patent application Ser. No. 937,973, the latter is affected by an evident limitation in terms of operative flexibility, as the humidification of the membranes can only be effected with pure water (at least demineralized), and that this same fluid is deputed to heat withdrawal. It is evident, therefore, that the operation of the generator according to such design in critical conditions of temperature, pressure and current density is affected by the impossibility of utilizing a cooling fluid with improved characteristics.
Patent Application No. PCT/EP 00/04476 discloses an efficient system for pre-humidifying the gaseous reactants to be supplied to a fuel cell stack by means of a device filled with a reticulated metallic material, simultaneously fed with the gas to be humidified and with a calibrated flow of water in the liquid state; this system results more compact and effective than the traditional gas bubblers, nevertheless it cannot perform alone also the cooling of the cell, because the partial evaporation of the water flow takes place far away from the active area of the cell, which is the point where heat is given off. Once supplied to the cells, the completely saturated gaseous flow prevents the evaporation of water from the membrane, the latter remaining thereby well hydrated, but only provided an external means for thermal regulation maintains the cell temperature constant.
The conventional devices for thermal regulation in fuel cell stacks are associated with weights, volumes and costs deriving from manufacturing complications although the use of metallic materials sensibly improves the thermal exchange efficiency. A commonly utilised system for withdrawing the heat of reaction is to provide grooves inside the bipolar plates, through which a suitable coolant is circulated; this implies that the resulting plate has a considerable thickness, in order to allow the housing of the grooves, and that it is complex and also expensive, due to the machining needed for its production. A more convenient solution consists in delimiting, thanks to a suitable gasket, a recess between two smooth and thin shells, put in electric contact by means of metallic inserts to form a bipolar plate; the recess may be conveniently exploited to circulate a coolant therein, and the resulting mechanical complication is limited. However, evident reasons of structural stability still impose a considerable total thickness for each bipolar plate. A method to cool the bipolar plates of a fuel cell stack with increased manufacturing simplicity is known from European Patent Application No. EP 0 896 379. The invention disclosed therein foresees the heat withdrawal from the bipolar plates of a fuel cell stack by circulating a cooling fluid in a peripheral region of such plates, external to the active area. In this way, the thickness of the plates can be drastically reduced, as no circulating means are provided inside such plates. This solution allows an acceptable performance when the process conditions are not too demanding, especially in terms of temperature and current density; in fact, the removal of heat through a peripheral portion of the plates implies that a transversal thermal gradient is set up upon the cell surface: in other words, the innermost region of each cell will be hotter that the outermost ones, and such a phenomenon will have a repercussion on the stability of the polymeric membrane, which will be subject to irregular thermal expansion and possibly to phenomena of local dehydration, which in their turn would result in a downright fall both of the ionic conductivity and of the mechanical properties. All of this introduces an undesirable limitation in cell design in terms of width of the active area, because the bigger is the distance between the centre and the periphery of the cell, the sharper will be the thermal gradient. Furthermore, in order to maintain the innermost region of the membrane at a temperature below the maximum allowed, it is compulsory that the outermost region be maintained at a very low temperature. This last factor causes, in its turn, two important drawbacks: first of all a considerable fraction of the two half-reactions would be carried out at a temperature undesirably low both in terms of catalyst activity and of membrane ionic conductivity; secondly, the circulation of a coolant at too low temperature would prove too demanding for the global economics of the systems, and even unfeasible for certain types of application (e.g. for the automotive traction).